March 24, 2000

Researchers Find Ocean Temperature Rising, Even in the Depths

By WILLIAM K. STEVENS

n important piece
of the global-warming picture has come into clearer focus with a confirmation
by scientists that the world's oceans have soaked up much of the warming of
the last four decades, delaying its full effect on the atmosphere and thus on
climate.

The warming of the deep oceans
had long been predicted, and the
consequent delaying effect long
thought to exist.

But until now the
ocean's heat absorption had not been
definitively demonstrated, and its
magnitude had not been determined.

The finding, by scientists at the
National Oceanographic Data Center
in Silver Spring, Md., is based on an
analysis of 5.1 million measurements, by instruments around the
world, of the top two miles of ocean
waters from the mid-1950's to the
mid-1990's.

The analysis, the first on
a global scale, is being published
today in the journal Science.

As the earth warms, from either
natural or human causes, or both, not
all the extra heat goes immediately
into the atmosphere, where its effect
on climate is most direct.

Much of it
is absorbed by the oceans, which
store it for years or decades before
releasing it.

This means that to whatever extent the planet is being warmed by
emissions of greenhouse gases like
carbon dioxide, which are produced
by the burning of coal, oil and natural
gas, only part of that heating has
materialized so far at and above the
earth's surface.

Some experts believe that about half the greenhouse
warming is still in the oceanic pipeline and will inevitably percolate to
the air in the decades just ahead.

The average surface temperature
of the globe has risen by about 1
degree Fahrenheit over the last 100
years.

Over the last 25 years, the rate
of surface warming has accelerated,
amounting to the equivalent of about
3.5 degrees a century.

By comparison, the world is 5 to 9 degrees warmer now than in the depths of the last
ice age, 18,000 to 20,000 years ago.

Scientists generally agree that it is
unclear how much of the warming is
attributable to greenhouse gases
and how much to natural causes;
many think both are involved.

The new study shows that the average warming of the seas over the
40-year study period amounted to
about one-tenth of a degree Fahrenheit for the top 1.9 miles of ocean
water as a whole, and more than half
a degree in about the top 1,000 feet.

It is possible that the ocean may
now be giving up to the atmosphere
some of the heat it stored in the early
part of the study period, but this has
not been established, said Sydney
Levitus, the chief author of the study.
He is the director of the Ocean Climate Laboratory, part of the data
center at Silver Spring, which in turn
is part of the National Oceanic and
Atmospheric Administration.

Likewise, Mr. Levitus said, it is
possible but not established that
more frequent appearances of the
phenomenon known as El Niño, a
semi-periodic warming of the eastern tropical Pacific that disrupts
weather around the world, are related to the generally warming ocean.

The magnitude of the oceanic
warming surprised some experts.
One, Dr. Peter Rhines, an oceanographer and atmospheric scientist at the
University of Washington in Seattle,
said it appeared roughly equivalent
to the amount of heat stored by the
oceans as a result of seasonal heating in a typical year.

"That makes it
a big number," he said.

Dr. James E. Hansen, a climate
expert at the NASA Goddard Institute for Space Studies in New York,
said the finding was important because, "in my opinion, the rate of
ocean heat storage is the most fundamental number for our understanding of long-term climate change."

Three years ago, Dr. Hansen and
colleagues used a computer model to
calculate the amount of warming
that should have been produced up
till then by external influences on the
climate system like greenhouse gases and solar radiation.

They found
that because of the storage of heat in
the ocean, only about half the surface
warming should have appeared.

Mr.
Levitus and his fellow researchers
say in their paper that their findings
support the Hansen conclusion.

Still, Mr. Levitus said the cause of
the oceanic warming was not clear,
although "I believe personally that
some of it is due to greenhouse gases."

Some scientists believe that natural factors like recurring oscillations
in ocean surface temperature in various parts of the world may play a
role in the last century's warming.
For example, studies by Dr. Gerard
Bond of Columbia University's Lamont Doherty Earth Observatory
found that the climate of the North
Atlantic region, at least, had alternated between cooler and warmer
every 1,500 years, more or less.

The world may be entering one of
the natural warming cycles now, say
Dr. Bond and Dr. Charles D. Keeling,
a climate expert at the Scripps Institution of Oceanography in San Diego.

In a study published this week in the
online edition of Proceedings of the
National Academy of Sciences, Dr.
Keeling suggested that a natural
fluctuation in ocean tides over hundreds of years might contribute to
these long-term cycles of warming
and cooling.